fix: prevent send-on-closed-channel panic in concurrent Put/Delete + Close (C7)

Put/Delete checked writeStopped then called queue.Submit (which sends on
a channel). Close set writeStopped then closed the channel. With no
synchronization between the check and Submit, a concurrent Close could
close the channel during the window, causing "send on closed channel"
panic in Put/Delete.

The CommitQueue.Close comment said "Callers must stop submitting before
Close" but Put/Delete didn't enforce this.

Changes:
- wal/writer.go: add submitMu sync.RWMutex. Put/Delete construct entry
  outside the lock, then hold RLock during writeStopped check + Submit;
  release before <-req.Result. Close holds write lock during
  writeStopped.Store + queue.Close, guaranteeing no Submit is in progress
  when the channel is closed.
- wal/writer_test.go: add TestConcurrentPutCloseNoPanic and
  TestConcurrentDeleteCloseNoPanic. 100 goroutines + 1 closer, recover
  panics. Run with -count=50 for regression detection.

Verified: all existing tests pass. New concurrent tests pass with
-count=50 (0 panics). go test -race ./... green.

Audit context: docs/audit-3.2.md C7.

wal/writer_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"sync"
+	"sync/atomic"
 	"testing"
 
 	"github.com/dailz/go-kv/config"
@@ -164,3 +165,101 @@ func TestWriteStoppedAfterIOError(t *testing.T) {
 
 	_ = ww.Close()
 }
+
+// -------- C7 regression guards --------
+
+// Regression guard for C7: concurrent Put + Close must not panic.
+// Buggy code has a race window between writeStopped check and Submit's
+// send on channel; Close closing the channel during that window causes
+// "send on closed channel" panic.
+//
+// Note: this test guarantees "no panic AFTER fix", not "must panic BEFORE
+// fix" — the race window is narrow (Put check → Submit, a few instructions)
+// and the default queue capacity (10000) means sends rarely block. Run
+// with -count=50 to increase regression detection probability.
+func TestConcurrentPutCloseNoPanic(t *testing.T) {
+	dir := t.TempDir()
+	cfg := config.Defaults()
+	ww, err := NewWalWriter(&cfg, dir, 0, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	const goroutines = 100
+	var wg sync.WaitGroup
+	var panicCount atomic.Int32
+	start := make(chan struct{})
+
+	for i := 0; i < goroutines; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			defer func() {
+				if r := recover(); r != nil {
+					panicCount.Add(1)
+				}
+			}()
+			<-start
+			_ = ww.Put([]byte("k"), []byte("v"))
+		}()
+	}
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		<-start
+		_ = ww.Close()
+	}()
+
+	close(start)
+	wg.Wait()
+
+	if panicCount.Load() > 0 {
+		t.Fatalf("concurrent Put + Close caused %d panic(s) — C7 regression",
+			panicCount.Load())
+	}
+}
+
+// Regression guard for C7: same as above but for Delete.
+func TestConcurrentDeleteCloseNoPanic(t *testing.T) {
+	dir := t.TempDir()
+	cfg := config.Defaults()
+	ww, err := NewWalWriter(&cfg, dir, 0, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	const goroutines = 100
+	var wg sync.WaitGroup
+	var panicCount atomic.Int32
+	start := make(chan struct{})
+
+	for i := 0; i < goroutines; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			defer func() {
+				if r := recover(); r != nil {
+					panicCount.Add(1)
+				}
+			}()
+			<-start
+			_ = ww.Delete([]byte("k"))
+		}()
+	}
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		<-start
+		_ = ww.Close()
+	}()
+
+	close(start)
+	wg.Wait()
+
+	if panicCount.Load() > 0 {
+		t.Fatalf("concurrent Delete + Close caused %d panic(s) — C7 regression",
+			panicCount.Load())
+	}
+}